Conventional arc lamps provide a high energy density, high intensity, sharply defined source which is desirable in a number of applications. The high energy density and high intensity of pulsed short arc flashlamps, otherwise known as bulb-type flashtubes, for example, are desirable in analytical applications such as absorption spectroscopy where the chemical sensitivity is a direct function of the energy density at the target sample, as well as applications such as fluorometry and HPLC applications. Flashlamps in general are arc lamps that operate in a pulsed mode and are capable of converting stored electrical energy into intense bursts of radiant energy covering the ultraviolet (UV), visible, and infrared (IR) regions of the spectrum. The combination of an unconfined arc and short arc length result in a low impedance device capable of producing microsecond pulse durations, typically between 0.7 and 10 μs. With these short pulse durations, flash repetition rates of up to 300 Hz are readily obtainable. The high level of flash-to-flash stability needed for many applications, such as instabilities of 0.25% or less, is obtained through the spatial stability of the arc discharge and the total spectral stability of the light output of the flashlamp. The time jitter typically is less than 150 ns, with a recovery time of the discharge on the order of about 150 μs. Descriptions of such arc lamps can be found, for example, in U.S. Pat. No. 6,274,970, which is hereby incorporated herein by reference.
FIG. 1 shows an exemplary short arc flashlamp 100 of the prior art having a lamp housing consisting of a cylindrical metal or glass enclosure 102, otherwise known as a can or envelop. A window 106 is positioned near a circular opening 104 at a transmitting end of the envelope. A stem 108 is secured at the opposite end of the envelop 102, and a weld or braze ring 110 can be used to connect the stem 108 to the envelop by a process such as arc welding. The sealed envelope can be filled with a pressurized, inert gaseous atmosphere, such as an atmosphere containing xenon gas at about 3 atm. Flashlamps are similar to other arc lamps in that optical radiation is produced when sufficient energy is transferred to the gas atoms to cause excitation and ionization. Xenon is used in most flashlamps since xenon is thought to be the most efficient of the inert gases for converting electrical energy to optical energy. Inside the sealed interior of the lamp are positioned an anode 112 and a cathode 114, connected by stem pins 116 and 118, respectively, to the stem 108. The stem pins pass through the stem to allow a voltage to be applied across the arc gap formed between the electrodes by an appropriate circuit (not shown). Current typically is supplied by a capacitor discharging large amounts of energy in a short period of time. The anode and cathode are configured to allow for an arc discharge in the sealed envelope when the capacitor in the circuit discharges across the gap.
The alignment of the electrodes is critical for efficiency and stability. At least one trigger probe 120 is positioned near the arc gap between the anode 112 and the cathode 114 to guide the arc. The trigger probe can be coupled with a trigger electrode 122 for passing a high voltage trigger pulse near the arc gap, creating a low impedance path between the anode and cathode such that the voltage capacitor can discharge across the gap. The number of trigger probes can depend on the arc length and type of flashlamp. A sparker electrode 124 is positioned inside the envelope for generating a preionization of the gas, in order to obtain a more uniform discharge. The discharge across the arc gap can generate light that is reflected by a mirror assembly 126 positioned relative to the arc gap and/or transmitted through the light transmitting window 106. The mirror assembly can have a cavity 128 made of a material such as stainless steel, copper, or glass, which can hold a drop-in reflector or have a material coating thereon acting as the reflector. The alignment of the mirror also can be critical for efficiency. The window assembly 126 also can include an exhaust pipe 128.
Many applications utilize external optical fibers to couple light from these lamps to the appropriate location. Bundles of glass or fused silica fibers are typically positioned adjacent the output window of the lamp to capture the transmitted light. This approach can be somewhat troublesome, however, as it can be difficult to precisely position the fiber bundle relative to the location of the discharge. This positioning can involve operating the lamp for a number of discharges and moving the input end of the fiber transversely across the window exit surface in order to find the optimal position, or “sweet spot,” relative to the discharge. This process can be time consuming, imprecise, and can lower the amount of manufacturing throughput. Further, such alignment may need to be readjusted due to movement or operation of the lamp.
Another problem with these existing arc lamps is the inherent instability. When a discharge pulse occurs between the two main electrodes, the resultant explosion, though somewhat controlled, will have certain fluctuations in parameters such as position and spectral intensity. The resultant instabilities can be propagated through the fiber bundle, and can produce an output beam of light that is unacceptable for many high-precision applications.
Further still, existing approaches to utilizing fiber optic illumination with an arc lamp source require elements such as an optical chain, at least one lens, and at least one optical filter to transfer the light from the lamp. At any of these optical elements, as well as at the window of the lamp itself, the illumination can experience various reflective and transmissive losses. These losses can have undesirable effects upon the end application, such as various biomedical applications